gating system design

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Lecture Notes of Chinmay Das

GATING SYSTEM DESIGN

Figure 1: Gating systems

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Elements of Gating System

The gating systems refer to all those elements which are connected with the flow of molten metal from the

ladle to the mould cavity. The elements of gating systems are

• Pouring Basin

• Sprue

• Sprue Base Well

• Runner

• Runner Extension

• Ingate

• Riser

Figure 2: Components of a gating system

Any gating system designed should aim at providing a defect free casting. This can be achieved by

considering following requirements.

• The mould should be completely filled in the smallest possible time without having to raise neither

metal temperature nor use of higher metal heads.

• The metal should flow smoothly into the mould without any turbulence. A turbulence metal flow

tends to form dross in the mould.

• Unwanted materials such as slag, dross and other mould materials should not be allowed to enter

the mould cavity.

• The metal entry into the mould cavity should be properly controlled in such a way that aspiration

of the atmospheric air is prevented.

• A proper thermal gradient should be maintained so that the casting is cooled without any

shrinkage cavities or distortions.

• Metal flow should be maintained in such a way that no gating or mould erosion takes place.

• The gating system should ensure that enough molten metal reaches the mould cavity.

• It should be economical and easy to implement and remove after casting solidification.

• The casting yield should be maximised.

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The liquid metal that runs through the various channels in the mould obeys the Bernoulli’s theorem which

states that the total energy head remains constant at any section. Ignoring frictional losses, we have

Where h = Potential Head, m

P = Static Pressure, Pa

v = Liquid Velocity, m / s

ρ g = w = Specific weight of liquid, N / m2

g = Acceleration due to gravity, m / s2

Though quantitatively Bernoulli’s theorem may not be applied, it helps to understand

qualitatively, the metal flow in the sand mould. As the metal enters the pouring basin, it has the highest

potential energy with no kinetic or pressure energies. But as the metal moves through the gating system, a

loss of energy occurs because of the friction between the molten metal and the mould walls. Heat is

continuously lost through the mould material though it is not represented in the Bernoulli’s equation.

Another law of fluid mechanics, which is useful in understanding the gating system behaviour, is

the law of continuity which says that the volume of metal flowing at any section in the mould is constant.

The same in equation form is

Q = A1V1 = A2V2

Where Q = Rate of flow, m3 / s

A = Area of cross section, m2

V = Velocity of metal flow, m / s

Pouring Time The main objective for the gating system design is to fill the mould in the smallest time. The time for

complete filling of a mould is called pouring time. Too long a pouring time requires a higher pouring

temperature and too less a pouring time means turbulent flow in the mould which makes the casting defect

prone. The pouring time depends on the casting materials, complexity of the casting, section thickness and

casting size. Steels lose heat very fast , so required less pouring time while for non-ferrous materials longer

pouring time is beneficial because they lose heat slowly and tend to form dross if metal is pour too quickly.

Ratio of surface area to volume of casting is important in addition to the mass of the casting. Also

gating mass is considered when its mass is comparable to the mass of the casting.

• For grey cast iron up to 450 Kg

Pouring time, t = K { 1.41 + 59.14

T } W seconds

Where K = Fluidity of iron in inches / 40

T = Average section thickness, mm

W = Mass of the casting, Kg

• For grey cast iron greater than 450 Kg

Pouring time, t = K { 1.236 + 65.16

T }

3 W seconds

Typical pouring times for cast iron are

Casting mass Pouring time in seconds

20 Kg 6 to 10

100 Kg 15 to 30

• Steel Casting

Pouring time, t = (2.4335 – 0.3953 log W) W seconds

• Shell moulded ductile iron( vertical pouring)

Pouring time, t = K1 W seconds

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Where K1 = 2.080 for thinner sections

= 2.670 for sections 10 to 25 mm thick

= 2.970 for heavier sections

• Copper alloy castings

Pouring time, t = K2 3 W seconds

Where K2 is a constant whose value is given by 1.30 for top gating, 1.80 for bottom gating, 1.90 for brass

and 2.80 for tin bronze.

Choke Area After calculation of pouring time, it is required to establish the main control area which meters the metal

flow into the mould cavity so that the mould is completely filled within the calculated pouring time. The

controlling area is the choke area. The choke area happens to be at the bottom of the sprue and hence the

first element to be designed in the gating system is the sprue size and its proportions. The main advantage

in having sprue bottom as the choke area is that proper flow characteristics are established early in the

mould.

The choke area can be calculated using Bernoulli’s equation as

A = gHdtC

W

2

Where A= Choke area, mm2

W= Casting mass, Kg

t = Pouring time, s

d = Mass density of the molten metal, Kg / mm3

g = acceleration due to gravity, mm /s2

H = Effective metal head ( sprue height), mm

C = Efficiency factor which is a function of the gating system used

The effective sprue height H , of the mould depends on the casting dimensions and type of the gating used.

It can be calculated using the following relations.

Top gate, H= h Bottom gate H = h - 2

c and H = h -

c

pxp

2

Where h = Height of the sprue

p = Height of mould cavity in cope

c = Total height of the mould cavity

Figure 3: Effective sprue height

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The efficiency coefficient of the gating system depends on the various sections that are normally

used in a gating system. The elements of a gating system should be circular in cross section since they have

lower surface area to volume ratio which would reduce heat loss and have less friction. Moreover,

streamlining the various gating elements would greatly increase volumetric efficiency of the gating system

and allow for smaller size gates and runners which would increase the casting yield. Whenever a runner

changes direction or joins with another runner or gate, there is some loss in the metal head, all of which

when taken properly into consideration would give the overall efficiency of the gating system.

Type of system Tapered choked sprue Straight sprue runner choke

Single runner 0.90 0.73

Two runners with multiple gates

no bends in runners 0.90 0.73

Two runners with multiple gates

900 bends in runners

0.85 0.70

Table I: Efficiency coefficients, C for various types of gating systems

Sprue The sprues should be tapered down to take into account the gain in velocity of the metal as it flows down

reducing the air aspiration. The exact tapering can be obtained by equation of continuity. Denoting the top

and the choke sections of the sprue by the subscripts t and c respectively, we get

AtVt = ACVC

Or At = AC t

c

V

V

Since the velocities are proportional to the square of the potential heads, then from Bernoulli’s equation

At = AC

t

c

h

h

The square roots suggest that the profile of the sprue should be parabolic if exactly done as per the above

equation. But making a parabolic sprue is inconvenient in practice and therefore a straight taper is

preferable.

Figure 4: Sprue and pouring basin height and area

Depth in pouring basin, mm Sprue height,

mm 50 100 150 200 250

50 1.414 1.225 1.155 1.118 1.095

100 1.732 1.414 1.291 1.225 1.183

150 2.000 1.581 1.414 1.323 1.265

200 2.236 1.732 1.528 1.414 1.342

250 2.450 1.871 1.633 1.500 1.414

375 2.915 2.179 1.871 1.696 1.581

500 3.317 2.450 2.082 1.871 1.732

600 3.742 2.739 2.309 2.062 1.897

Table II: Theoretical ratios of sprue top and choke areas based on pouring basin depth

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Other Gating Elements

Pouring Basin The main function of a pouring basin is to reduce the momentum of the liquid flowing into the

mould by settling first into it. In order that the metal enters into the sprue without any turbulence it is

necessary that the pouring basin be deep enough, also the entrance into the sprue be a smooth radius of at

least 25 mm. The pouring basin depth of 2.5 times the sprue entrance diameter is enough for smooth metal

flow and to prevent vortex formation. In order that vortex is not formed during pouring, it is necessary that

the pouring basin be kept full and constant conditions of flow are established. This can be achieved by

using a delay screen or a strainer core. A delay screen is a small piece of perforated thin tin sheet placed in

the pouring basin at the top of the down sprue. This screen usually melts because of the heat from the metal

and in the process delays the entrance of metal into the sprue thus filling the pouring basin fully. This

ensures a constant flow of metal as also exclude slag and dirt since only metal from below is allowed to go

into the sprue. A similar effect is also achieved by a strainer core which is a ceramic coated screen with

many holes. The strainer restricts the flow of metal into the sprue and thus helps in quick filling of the

pouring basin. Pouring basins are most desirable for alloys which form troublesome oxide skins

(aluminium, aluminium bronze, etc.)

Figure 5: Pouring basin (1)

Figure 6: Pouring basin (2)

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Sprue Base Well The provision of a sprue base well at the bottom of the sprue helps in reducing the velocity of the incoming

metal and also the mould erosion. A general guide line could be that the sprue base well area should be five

times that of the sprue choke area and the well depth should be approximately equal to that of the runner.

Figure 7: Sprue base well design

Gating Ratios It refers to the proportion of the cross sectional areas between the sprue, runner and ingates and is

generally denoted as sprue area : runner area : ingate area. Depending on the choke area there can be two

types of gating systems:

• Non-pressurised

• Pressurised

A non –pressurised gating system having choke at the sprue base, has total runner area and ingate area

higher than the sprue area. In this system there is no pressure existing in the metal flow system and thus it

helps to reduce turbulence. This is particularly useful for casting drossy alloys such as aluminium alloys

and magnesium alloys. When metal is to enter the mould cavity through multiple ingates, the cross section

of the runner should accordingly be reduced at each of a runner break-up to allow for equal distribution of

metal through all ingates. A typical gating ratio is 1:4:4

The disadvantages of unpressurised gating are:

• The gating system needs to be carefully designed to see that all parts flow full. Otherwise some

elements of the gating system may flow partially allowing for the air aspiration. Tapered sprues

are invariably used with unpressurised system. The runners are maintained in drag while the gates

are kept in cope to ensure that runners are full.

• Casting yield gets reduced because of large metal involved in the runners and gates.

In the case of pressurised gating system normally the ingates area is the smallest, thus maintaining a back

pressure throughout and generally flows full and thereby, can minimize the air aspiration even when a

straight sprue is used. It provided higher casting yield since the volume of metal used up in the runners and

gates is reduced. Because of turbulence and associated dross formation, this type of gating system is not

used for light alloys but can be advantageously used for ferrous castings. A typical gating ratio is 1:2:1.

While designing the runner system, care should be taken to reduce sharp corners or sudden change

of sections since they tend to cause turbulence and gas entrapment. Though from heat loss factor circular

cross section runners are preferable, traditionally trapezoidal runner sections are employed to reduce the

turbulence. The approximate proportions are fro a square to rectangle with width twice as that of the depth

of the runner. When multiple ingates are used, the runner cross section should be suitably restricted at the

separation of each runner in the interest of uniform flow through all sections.

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It is a general practice to cut runner in the cope and the ingate in the drag to help in the trapping of

the slag. Sometimes it is good to have half of the runner in the cope side and rest in the drag.

Figure 8: Runners

But for aluminium alloy castings, it is recommended that the runners be placed in the drag and the ingates

in the cope so that dross (3.99 g/cm2) which is heavier compared to aluminium (2.70 g/cmm

2) is restricted.

Also the entry into runners from sprue base well should be made as smooth as possible in such castings,

otherwise the direction of flow would tend to be turbulent and leads to drossing when any change abruptly

occurs in the cross sectional areas.

Material Gating Ratio

Aluminium 1:2:1, 1:1.2:2, 1:2:4, 1:3:3, 1:4:4, 1:6:6

Aluminium bronze 1: 2.88:4.8

Brass 1:1:1, 1:1:3, 1.6:1.3:1

Copper 2:8:1, 3:9:1

Ductile iron 1.15:1.1:1, 1.25:1.13:1, 1.33:2.67:1

Grey cast iron 1:1.3:1, 1:4:4, 1.4:1.2:1, 2:1.5:1, 2:1.8:1, 2:3:1, 4:3:1

Magnesium 1:2:2, 1:4:4

Malleable iron 1:2:9.5, 1.5:1:2.5, 2:1:4.9

Steels 1:1:7, 1:2:1, 1:2:1.5, 1;2:2, 1:3:3, 1.6:1.3:1

Table III: Some gating ratios used in practice

Ingate The ingate can be considered as a weir with no reduction in cross section of the stream at the gate. Then the

rate of flow of molten metal through the gates depends on the free height of the metal in the runner and the

gate area & the velocity with which metal is flowing in the runner. The free height, h can be calculated as

h = 1.6 3

gxbxb

QxQ +

2g

VxV mm

Where Q = metal flow rate, mm3/s

b = gate width, mm

V = metal velocity in runner, mm/s

g = acceleration due to gravity, mm/s2

Having obtained the head of metal, the height of the gate h, is given by

h1 = h – 5 mm

Gates higher than this will not fill completely and those lower than this will increase the velocities of the

stream entering into. The ingates are generally made wider compared to depth, up to a ratio of 4. This

facilitates in the severing of the gates from the casting after solidification. It may sometimes preferable to

reduce the actual connection between the ingate and the casting by means of a neck-down so that the

removal of it is simplified. The following points should be kept in mind while choosing the positioning of

the ingates.

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• Ingate should not be located near a protruding part of the mould to avoid the striking of vertical

mould walls by molten metal stream.

• Ingates should be preferably be placed along the longitudinal axis of the mould wall.

• It should not be placed near a core print or a chill.

• Ingate cross sectional area should preferably be smaller than the smallest thickness of the casting

so that the ingates solidify first and isolate the casting from the gating system. This would reduce

the possibility of air aspiration through gating system in case of metal shrinkage.

• It is possible that the farthest gate from the sprue is likely to flow more metal than others,

particularly in the case of unpressurised system. To make for more uniform flow through all the

gates, the runner area should be reduced progressively after each ingate, such that restriction on

the metal flow would be provided.

Figure 9: Multiple ingates feeding the various parts of a casting

Figure 10: Multiple ingates designed to induce uniform flow through all gates

Slag Trap Systems In order to obtain sound casting quality, it is essential that the slag and other impurities be removed from

the molten metal fully before it enters the mould cavity. Apart from the use of pouring basins and strainer

cores the following methods are also used.

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Runner Extension: Normally the metal which moves first into the gating system is likely to contain slag

and dross which should not be allowed to get into the mould cavity. This could be achieved by extending

the runner beyond the ingates so that the momentum of the metal will carry it past the gates and to a blind

alley called runner extension. A runner extension having a minimum of twice the runner width is desirable.

Whirl Gate: Another method employed successfully to trap the slag from entering steel casting is a whirl

gate. This utilizes the principle of centrifugal action to throw the dense metal to the periphery and retain the

lighter slag at the centre. In order to achieve this action, it is necessary that entry area should be at least 1.5

times the exit area so that the metal is built up at the centre quickly. Also the metal should revolve 2700

before reaching the exit gate so as to gain enough time for separating the impurities.

Figure 11: Whirl gate

Design of Riser The function of a riser (also called reservoir, feeders, or headers) is to feed the casting during solidification

so that no shrinkage cavities are formed. The requirement of risers depends to a great extent upon the type

of metal poured and the complexity of the casting. Let us consider the mould of a cube which is filled with

liquid metal. As time progresses, the metal starts losing heat through all sides and as a result starts freezing

from all sides equally trapping the liquid metal inside. But further solidification and subsequent volumetric

shrinkage and the metal contraction due to change in temperature causes formation of a void. The

solidification when complete, finally results in the shrinkage cavity as shown in the figure. The reason for

the formation of the void in the cube casting is that the liquid metal in the centre which solidifies in the end

is not fed during the solidification; hence the liquid shrinkage ends up as a void. Such isolated spots which

remain hot till the end are called hot spots.

Figure 12: Solidification of cube casting

Functions of Risers

• Provide extra metal to compensate for the volumetric shrinkage

• Allow mold gases to escape

• Provide extra metal pressure on the solidifying metal to reproduce mold details more exactly.

• To compensate mould expansion during pouring of hot liquid metal because of soft mould.

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It is the task of casting designer to reduce all hot spots so that no shrinkage cavities occurred. Since

solidification of the casting occurs by loosing heat from the surfaces and the amount of the heat is

given by the volume of the casting, the cooling characteristics of a casting can be represented by the

surface area to the volume ratio. Since the riser is almost similar to the casting in its solidification

behaviour, the riser characteristics can also be specified by the ratio of its surface area to volume. If

this ratio of casting is higher, then it is expected to cool faster.

According to Chvorinov, solidification time can be calculated as

ts = K { SA

V }

2

Where ts = solidification time, s

V = volume of the casting,

SA = surface area

K = mould constant which depends on pouring temperature, casting & mould thermal

Characteristics

The freezing ratio, X of a mould is defined as the ratio of cooling characteristics of casting to that of the

riser.

X = VriserSAriser /

Vcasting / SAcasting

In order to feed the casting, the riser should solidify last and hence its freezing ratio should be greater than

unity.

CAINE’s Method

X = { a / Y-b} + c Where Y = riser volume / casting volume

a, b, c are constants whose values for different materials are given here.

Material a b c

Steel 0.10 0.03 1.00

Aluminium 0.10 0.06 1.08

Cast iron, Brass 0.04 0.017 1.00

Grey cast iron 0.33 0.030 1.00

Aluminium bronze 0.24 0.017 1.00

Silicon bronze 0.24 0.017 1.00

Table IV: Values of a,b,c for different materials

Design Requirements of Risers

1. Riser size: For a sound casting riser must be last to freeze. The ratio of (volume / surface area)2

of the riser must be greater than that of the casting. However, when this condition does not meet, the metal

in the riser can be kept in liquid state by heating it externally or using exothermic materials in the risers.

2. Riser placement: the spacing of risers in the casting must be considered by effectively

calculating the feeding distance of the risers.

3. Riser shape: cylindrical risers are recommended for most of the castings as spherical risers,

although considers as best, are difficult to cast. To increase volume/surface area ratio the bottom of the riser

can be shaped as hemisphere.

Reference:

1. Manufacturing Technology by P.N.Rao, TMH, page126 to 179

gating system design

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